This invention relates to a sensitive method of quantitative analysis and more particularly to the analysis of the surface composition of samples containing trace amounts of one or more components by a highly efficient, direct detection of multiphoton ionized atoms representing the component or components of interest. In some embodiments, it is capable of detecting a component at levels in the range of parts per billion (ppb) and may require a sample as small as one atomic monolayer or less.
Recent trends in product specification, exemplified by stringent requirements in semiconductor processing, illustrate the importance of quantitative surface analysis of impurities at progressively lower impurity levels. In general, the development of instruments and other apparatus useful for this purpose has progressed through secondary ion mass spectroscopy (SIMS) and laser fluorescence spectroscopy (LFS) to multiphoton resonance ionization spectroscopy (RIS). In each instance, portions of a sample are removed by sputtering with an ion beam to produce secondary ions and neutral atoms in addition to backscattered primary ions. One of the limitations of the SIMS apparatus is that the detection usually involves only a small fraction of the sputtered flux leaving the surface in an ionized state and does not sample directly the major portion of the flux consisting primarily of neutral atoms. This limited detection plus the use of a mass spectrometer results in the need for a significant quantity of sputtered flux to be generated for the desired detection. With LFS, neutral atoms are laser excited and their fluorescence detected. The laser is tuned so that its frequency matches that of a transition of the sputtered atom. However, LFS in general has the disadvantage of low detection efficiencies such that only about 10.sup.-2 -10.sup.-4 photons per fluorescent photon arrive at the detector.
The development of RIS has provided an improvement in the detection methods associated with SIMS and LFS. In RIS, multiphoton ionization of neutral atoms provides a source of ionized neutrals for detection. Further details of an RIS apparatus are set forth in U.S. Pat. No. 4,442,354 which is hereby incorporated herein by reference. While RIS provides a detection method with extreme sensitivity, improvement in the collection efficiency of its ion detection system and reduction in background noise would increase its value for quantitative analysis. In particular, the desired ion detection system would provide improved discrimination between the signal due to multiphoton ionization of sputtered neutral atoms and noise sources primarily due to backscattered primary ions (charged or neutralized) and secondary ions produced in the sputtering process.
Accordingly, one object of the invention is the detection method based on RIS with improved efficiency. A second object of the invention is a detection method having reduced background noise. A third object of the invention is a detection method which does not require mass spectroscopy. Another object of the invention is a sensitive detection method capable of detection in the ppb to ppt range. Yet another object of the invention is detection method requiring a sample containing only a few atoms. These and other objects of the invention will become apparent from the following detailed description.